Leak from Roof

I have a flat roof. The leak starts in the ceiling alongside the wall I share with my next door neighbor and progresses along the tape that connects the ceiling sheet rock. The roofer that did the roof (2005) insists that it isn’t the roof but a pipe that lies along the party wall. The pipe was broken, so my next door neighbor had it fixed (with an elbow so it faces down; it isn’t blocked off completely in case it does some kind of ventilation). The water only drips from the ceiling. The walls are dry . Any ideas? Any references for roofers that know what they are doing? Many thanks!